Ontogenetic trajectories in the ornithischian endocranium.

Understanding ontogenetic and developmental patterns is critical for reconstructing the life history of fossil vertebrates. In dinosaurs, ontogenetic studies have nearly exclusively focused on changes in the cranial and post-cranial skeleton, whereas ontogenetic changes in the endocranium have received little attention. Here, we present digital reconstructions of the brain and inner ear anatomy of two ontogenetic stages of the Jurassic ornithischian dinosaur Dysalotosaurus lettowvorbecki. Results show that the endocranial anatomy underwent considerable changes during growth, including a rostrocaudal elongation of the olfactory apparatus, a reduction in the cephalic and pontine flexure and an increase in cerebellum size. Functional elements, such as the cerebral hemispheres and the inner ear, were already well developed in early ontogenetic stages, indicating a large degree of precociality. The anisotropic pattern of size and shape changes in the endocranium further indicates that ontogenetic trajectories may be controlled by functional and environmental demands in the different growth stages in Dysalotosaurus lettowvorbecki. The occurrence of similar ontogenetic patterns in the endocranial anatomy of derived ornithopod dinosaurs suggests a more widespread distribution of this growth trajectory.